Plume debates

Jason Morgan’s recognised in the early 1970-s that chains of volcanic islands and seamounts, such as the Hawaii-Emperor Chain, which cross sea-floor magnetic stripes, might have resulted from mantle “hot spots” that are fixed relative to motions of lithospheric plates. He went on to suggest that such magmatic anomalies might reflect narrow thermal upwellings within the deep mantle, and applied the term “plumes” to these notional convective zones. Geochemists have since flocked to active and extinct manifestations of within-plate magmatism, and developed a whole sub-culture of classification and hypotheses concerning their origin and inner workings. By the end of the 1990s over 5000 candidates for underlying plumes had been proposed, some still active and others inferred for past events, such as flood basalt provinces. Processing of seismic signals using supercomputers over the last few years has used them to map variations in P- and S-wave speeds at different depths in the mantle. Speeds below those expected are likely to reflect hot mantle relative to high-speed, colder regions. So seismic tomography potentially charts hot rising mantle and cool, descending parts; seemingly ideal for detecting mantle plumes and how deep they extend. Early results centred on proposed plumes were a mixed bag. Some seemed to have very deep origins, perhaps down to the core-mantle boundary, whereas others appeared to be above hardly anomalous mantle. Most exciting was a zone of hot, probably rising mantle with a source at the top of the core beneath the South Atlantic, yet whose upper parts sloped obliquely upwards towards the Red Sea. It seemed that the Afar plume, believed to have been responsible for continental flood volcanism in Kenya and the Ethiopian Plateau, and perhaps the East African Rift and opening of the Red Sea, still existed. Hot-spot activity is a minor aspect of global tectonics today, so it is not an ideal time to ponder on plumes. If they are real, then periods of massive flood volcanism would have been responses to superplumes, but the last in Ethiopia was 30 Ma ago.

Exciting as seismic tomography is, its resolution is currently too coarse to pick out the most revealing features of the plumes that potentially it could detect. To have sufficient gravitational potential energy to rise through the entire mantle, a very large volume is required, and that is assigned to the “plume head”. Some hotspots are over large volumes of hot mantle, but they lie just beneath the lithosphere, and could have their origin at any level in the mantle. The tracks that they followed, if any, and which might continue to be a conduit for uprising material would be much narrower. Such predicted “plume tails” are too small for resolution by current tomography. A compilation and re-classification of hot spots (Courtillot, V. et al. 2003. Three distinct types of hotspots in the Earth’s mantle. Earth and Planetary Science Letters, v. 205, p. 295-308) has whittled down candidates for mantle plumes to a mere 50 or so, with less than 10 likely to have risen from core depths. Two responses have arisen about this hugely popular topic: that Morgan’s ideas are still basically valid, but need more work (DePaulo, D.J. & Manga, M. 2003. Deep origin of hotspots – the mantle plume model. Sciene, v. 300, p. 920-921); that hotspots might be linked to plate tectonics, and that mantle plumes are nothing more than a “belief system” (Fouger, G.R. & Natland, J.H. 2003. Is “hotspot” volcanism a consequence of plate tectonics? Science, v. 300, p. 921-922). A sensible aim that might resolve matters is to seek materials from the largest magmatic events – flood basalts – that should contain unambiguous geochemical signs that their parent mantle was at some stage exchanging matter with the core, if they had formed after rise of a superplume. But, every line of approach to deep-mantle processes relies on proxy evidence, several steps removed from actual events and properties. That makes David Stephenson’s proposal for a mission to the core (above) so urgently in need of support!

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